This invention relates generally to molding thermoplastic materials and more particularly concerns a multilayered mold structure having active heating means and an improved insulating layer.
Molding of thermoplastic resins is a promising technique for producing a wide variety of plastic parts. Depending on specific requirements, such plastic parts can be made by any of a number of molding processes such as blow molding, compression molding and injection molding. One important requisite for the use of molded plastic parts, particularly in large, exterior panel applications, is a smooth surface quality. The surface of molded plastic parts should be as smooth as that of current exterior parts made from sheet metal. However, conventional molding of plastic parts tends to produce various surface imperfections that require labor intensive sanding and polishing operations to correct. Injection molding of resins with fillers in the form of fibers or powders is particularly susceptible to such imperfections. Molds for injection molding of thermoplastic resin are usually made from metal material such as iron, steel, stainless steel, aluminum alloy or brass. Such materials are advantageous in that they have high thermal conductivity and thus allow the melt of thermoplastic resin to cool rapidly and shorten the molding cycle time. However, because of the rapid cooling, the injected resin freezes instantaneously at the mold surface. The freezing of these materials at the mold surfaces creates rough surfaces such as exposed fillers, voids and porosity. Processing difficulties arise when producing thin, large parts. The quick solidification of the melt combined with limited flowability of the materials makes it difficult to achieve melt flow over a large area. The use of multiple gates for large and/or complex mold cavities produces weld lines, which are unsightly and weak. Another important issue in injection molding of high quality parts is the residual stresses in the molded parts. Residual stress inside a part can result in dimensional instability over the lifetime of the part. Nonuniform residual stresses also produce differential refractive indices. The dimensional stability and uniformity of the refractive index are critically required for high quality optical parts.
An approach to improve the surface quality of plastic parts produced by compression molding and blow molding has been disclosed in the above mentioned applications Ser. No. 07/435,639 and 07/435,640, respectively. These applications generally disclose molding structure in which an insulating layer is disposed on the mold core and a thin skin layer is disposed on the insulating layer. Due to the insulation, the skin layer retains heat during the molding operation, thereby avoiding the surface irregularities created by rapid surface cooling. Thus, these devices provide a smooth surface while maintaining a relatively short cycle time. Similarly, U.S. Pat. No. 4,225,109 to Yotsutsuji et al. discloses using a thermal insulating layer in an injection molding apparatus. U.S. Pat. No. 4,225,109 discloses using an insulating layer comprising a thermosetting resin, a hardener or curing agent and at least one of an inorganic filler, a metal powder filler and a fibrous filler.
The present invention addresses a problem existing in molding processes employing thermal insulation layers of the type discussed above. In particular, layer delamination may occur at the interface between the insulation layer and the mold core and/or the interface of the skin layer and the underlying insulating layer in cases where a skin layer is employed. It is believed that delamination occurs because of poor adhesion between the different layers and due to the differences in the coefficient of thermal expansion (CTE) of the materials of the different layers. Delamination is a prevalent problem in insulated molds because the typically plastic insulating layers have poor adhesion to the adjacent metal layers and a higher CTE than the core which is usually an iron or aluminum alloy.